Internet Protocol Multimedia Subsystem (“IMS”) is considered to be a technology enabler that brings the ability to deliver Internet Protocol (“IP”) based real-time person-to-person multimedia communication including voice-over-IP (“VoIP”) communication. IMS has a capability to interact with different services and applications, and also to easily set up multiple services in a single session as well as multiple simultaneous synchronized sessions for a user. In Universal Mobile Telecommunications System (“UMTS”), there are four classes of quality of service (“QoS”), or traffic classes, for data traffic as described in 3GPP TS 23.107 V6.0.0, entitled “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Quality of Service (QoS) concept and architecture (Release 6)” published by the 3rd Generation Partnership Project (“3GPP”), December 2003, which is herein incorporated by reference. The four QoS classes are: conversational class; streaming class; interactive class; and background class. The conversational class is generally used for telephony speech for real time conversation between live end-users, for example in Global System for Mobile Communications (“GSM”), and may include new applications such as VoIP and video conferencing tools. Because a failure to provide low enough transfer delay will result in unacceptable lack of quality to the end-users, there is a strict limit in acceptable transfer delay for the conversational class. Such IMS voice calls are processes using Adaptive Multi-Rate (“AMR”) speech voice encoder/decoder (“codec”) frame structure, as described in 3GPP TS 26.101 V5.0.0, entitled “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Mandatory speech codec speech processing function; Adaptive Multi-Rate (AMR) speech codec frame structure (Release 5)” published by the 3rd Generation Partnership Project (“3GPP”), June 2002, which is herein incorporated by reference. IMS voice calls are transported over the packet switched (“PS”) domain.
Each frame is divided into three parts, which are: AMR Header, AMR Auxiliary Information, and AMR Core Frame. The AMR Header includes the Frame Type and the Frame Quality Indicator fields. The AMR auxiliary information includes the Mode Indication, Mode Request, and Codec Cyclic Redundancy Check (“CRC”) fields. The AMR Core Frame includes the speech parameter bits, which are produced by the speech encoder, and are ordered according to their subjective importance. The speech parameter bits are further divided into three classes, Class A, Class B, and Class C according to their subjective importance. Class A contains bits that are most sensitive to errors, and any error in these bits is likely to cause a corrupted speech frame. Class A bits are protected by the Codec CRC in AMR Auxiliary Information. Classes B and C contain bits in which increasing error rates will gradually reduce the speech quality. However, decoding of an erroneous speech frame in Class B or C is usually possible without producing significant degradation. Class B bits are more sensitive to errors than Class C bits. These three classes of bits are subjected to different error protection.
An AMR frame is formed as a combination of AMR Header, AMR Auxiliary Information, and AMR Core Frame in a predetermined arrangement. Although AMR Core Frame may include bits of different classes subject to different error protection, the AMR frame is subjected to a fixed rate speech codec when the AMR frame is transported in the PS domain for real-time packet data.